Method and apparatus for searching for a global path

ABSTRACT

Some embodiments of the invention provide a method of searching for a global path between first and second sets of routable elements in a region of a layout. The method partitions the region into several rectangular sub-regions. It then identifies a set of sub-regions that contain the two sets of elements. Next, it performs a path search to identify a set of path expansions between a sub-region that contains a first-set element and a sub-region that contains a second-set element. When the method performs the path search, it explores expansions along non-Manhattan directions between the sub-regions.

CLAIM OF BENEFIT TO PRIOR APPLICATION

[0001] This patent application claims the benefit of U.S. ProvisionalPatent Application No. 60/427,131, filed Nov. 18, 1902.

FIELD OF THE INVENTION

[0002] The invention is directed towards a method and apparatus forsearching for a global path.

BACKGROUND OF THE INVENTION

[0003] An integrated circuit (“IC”) is a device (e.g., a semiconductordevice) that includes many electronic components, such as transistors,resistors, diodes, etc. These components are often interconnected toform multiple circuit components, such as gates, cells, memory units,arithmetic units, controllers, decoders, etc. An IC includes multiplelayers of wiring that interconnect its electronic and circuitcomponents. Traditionally, IC's use preferred direction (“PD”) wiringmodels, which specify a preferred wiring direction for each of theirwiring layers. In preferred direction wiring models, the preferreddirection typically alternates between successive wiring layers. Oneexample of a PD wiring model is the PD Manhattan wiring model, whichspecifies alternating layers of preferred-direction horizontal andvertical wiring.

[0004] Design engineers design IC's by transforming logical or circuitdescriptions of the IC's into geometric descriptions, called layouts. IClayouts typically include (1) circuit modules (i.e., geometricrepresentations of electronic or circuit IC components) with pins, and(2) interconnect lines (i.e., geometric representations of wiring) thatconnect the pins of the circuit modules. A net is typically defined as acollection of pins that need to be connected. A list of all or some ofthe nets in a layout is referred to as a net list.

[0005] To create layouts, design engineers typically use electronicdesign automation (“EDA”) applications. These applications provide setsof computer-based tools for creating, editing, and analyzing IC designlayouts. One EDA tool is a router that defines routes for interconnectlines that connect the pins of nets. Routing is generally divided intotwo phases: global routing and detailed routing. For each net, globalrouting generates a “loose” route for the interconnect lines that are toconnect the pins of the net. The “looseness” of a global route dependson the particular global router used. After global routes have beencreated, the detailed routing creates specific individual routes foreach net.

[0006] While some commercial global routers today might allow anoccasional diagonal jog, these routers do not typically explore diagonalrouting directions consistently when they are specifying the routinggeometries of the interconnect lines. This, in turn, increases the totalwirelength (i.e., total length of interconnect lines) needed to connectthe nets in the layout. Therefore, there is a need for a routing methodand apparatus that considers diagonal routing directions. There is alsoa need for a new way of identifying and costing routes.

SUMMARY OF THE INVENTION

[0007] Some embodiments of the invention provide a method of searchingfor a global path between first and second sets of routable elements ina region of a layout. The method partitions the region into severalrectangular sub-regions. It then identifies a set of sub-regions thatcontain the two sets of elements. Next, it performs a path search toidentify a set of path expansions between a sub-region that contains afirst-set element and a sub-region that contains a second-set element.When the method performs the path search, it explores expansions alongnon-Manhattan directions between the sub-regions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The novel features of the invention are set forth in the appendedclaims. However, for purpose of explanation, several embodiments of theinvention are set forth in the following figures.

[0009]FIG. 1 illustrates a 4×4 section of a congestion grid.

[0010]FIG. 2 illustrates a section of a length grid that divides eachGcell created by the congestion grid into four nodes.

[0011]FIG. 3 illustrates the four nodes in each Gcell on a particularlayer.

[0012] FIGS. 4-7 illustrate the directions of edges on interconnectlayers 2-5 in some embodiments of the invention.

[0013]FIG. 8 illustrates edges that cross the Gcells created by thecongestion grid.

[0014] FIGS. 9-12 illustrate four examples of internal zigs between thefour nodes of a Gcell.

[0015]FIG. 13 presents a three-dimensional side view of the exampleillustrated in FIG. 9.

[0016] FIGS. 14-21 illustrate eight examples of external zigs betweenthe four nodes of a particular Gcell and the eight nodes in four Gcellsthat are adjacent to the particular Gcell.

[0017]FIG. 22 presents a three-dimensional side view of the exampleillustrated in FIG. 14.

[0018]FIG. 25 illustrates a process that conceptually represents theoverall flow of the router in some embodiments of the invention.

[0019]FIGS. 26A, 26B, and 26C present three examples that illustrate howsome embodiments compute the capacity of a congestion edge between twoGcells on a given layer.

[0020]FIG. 27 illustrates a route-generation process that the routingprocess uses to generate a route for a particular net in someembodiments of the invention.

[0021]FIG. 28 presents one example of shadow nodes.

[0022]FIG. 29 illustrates a path-generation process that theroute-generation process uses in some embodiments.

[0023]FIG. 30 illustrates an example of a back trace operation used bythe path-generation process of FIG. 29.

[0024]FIG. 31 conceptually illustrates a computer system with which oneembodiment of the invention is implemented.

[0025]FIG. 32 illustrates an example of a multi-layer global route thatis produced by a router of some embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] In the following description, numerous details are set forth forpurpose of explanation. However, one of ordinary skill in the art willrealize that the invention may be practiced without the use of thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form in order not to obscure the descriptionof the invention with unnecessary detail.

[0027] Several embodiments of the invention provide a router that routesa set of nets in a region of an integrated circuit (“IC”) layout. Eachrouted net includes a set of routable elements in the IC-layout region.The routable elements are pins in the embodiments described below,although they might be other elements in other embodiments.

[0028] In the embodiments described below, the router uses a five-layerwiring model that has horizontal wiring on wiring layer 1, verticalwiring on wiring layer 2, horizontal wiring on wiring layer 3, +45°diagonal wiring on wiring layer 4, and −45° diagonal wiring on wiringlayer 5. One of ordinary skill will realize that the router can useother wiring models in other embodiments. In some embodiments, a line is“diagonal” if it forms an angle other than 0° or 90° with respect to thelayout's Cartesian coordinate axes, which are typically parallel withthe layout's boundary and/or the boundary of the layout's expected IC.On the other hand, an interconnect line is “horizontal” or “vertical” ifit forms an angle of 0° or 90° with respect to one of the coordinateaxes of the layout.

[0029] In the embodiments described below, the router partitions anIC-layout region into several square sub-regions. For each net beingrouted, the router then identifies a global route that connects the setof sub-regions that contain at least one pin of the net. Each net'sglobal route is a set of edges (i.e., interconnect lines) that connectsthe set of sub-regions that contain the net's pins. The identifiedroutes might have horizontal, vertical, and ±45° diagonal edges in theembodiments described below.

[0030] In these embodiments, the edges that are used to define eachroute are part of a routing graph used by the router. Section I providesan overview of this routing graph. Next, Section II provides the overallflow of the router. Section III then describes route-generation andpath-generation processes used by the router. Section IV describes acomputer system that can be used to implement some embodiments of theinvention.

[0031] I. Routing Graph, Congestion Grid, and Length Grid

[0032] In some embodiments, the router uses two grids to create arouting graph. The first grid is a coarser grid that divides the IClayout into a number of sub-regions, called Gcells. The second grid is afiner grid that divides each Gcell into four sub-regions. In theembodiments described below, the Gcells are square. This shape wellsupports ±45° routing, as any set of ±45° wiring tracks that cut througha square Gcell will fill its horizontal and vertical boundariesconsistently. One of ordinary skill will realize that other embodimentsmight use different shaped Gcells.

[0033] On each wiring layer, each of the four sub-regions in each Gcellis represented by a node at the center of the sub-region. Theembodiments described below use the coarser grid to measure routecongestion in the layout region, and use the finer grid to measure routelengths. Accordingly, below, the coarser grid is referred to as thecongestion grid, while the finer grid is referred to as the length grid.

[0034]FIGS. 1 and 2 illustrate small sections of the congestion andlength grids. As shown in these figures, intersecting horizontal andvertical lines form both these grids. FIG. 1 illustrates a 4×4 sectionof the congestion grid 100. This section divides a portion of an ICregion into 16 Gcells 105. In the embodiments described below, thecongestion grid divides the IC region into many more Gcells (e.g., tensor hundreds of thousands).

[0035]FIG. 2 illustrates a section of the length grid 200 thatcorresponds to the section of the congestion grid 100 illustrated inFIG. 1. As shown in this figure, the length grid divides each Gcell 105into four nodes 205 on each wiring layer. FIG. 3 illustrates the fournodes in each Gcell on a particular layer. There are a number of planarand non-planar edges between the nodes defined by the length grid 200.These edges are referred to as “node edges” in the discussion below.

[0036] A. Planar Edges

[0037] A planar node edge connects two adjacent routing-graph nodes.Each such edge represents a set of wiring tracks along the edge'sparticular direction that connect the two sub-regions represented by theedge's two nodes. Planar node edges have different directions ondifferent wiring layers. FIGS. 4 through 7 illustrate the directions ofthese edges on layers 2-5 in some embodiments. Some embodiments assumethat there are no planar node edges between routing-graph nodes on layer1, as this layer is often quite congested. Some of these embodimentspromote all the pins on layer 1 to layer 2. Other embodiments, however,specify planar node edges on layer 1. In some of these embodiments, theplanar node edges on layer 1 are in the same direction as node edges onlayer 3.

[0038]FIG. 4 illustrates that on layer 2 a vertical node edge 405 existsbetween each pair of vertically adjacent nodes, while FIG. 5 illustratesthat on layer 3 a horizontal node edge 505 exists between each pair ofhorizontally adjacent nodes. FIGS. 6 and 7 illustrate that on layers 4and 5, ±45° diagonal node edges exist only between certain pairs ofdiagonally adjacent nodes. Specifically, FIG. 6 illustrates that 45°diagonal node edges exist between northwest nodes 605 and southeastnodes 610 of different Gcells. As shown in this figure, no 45° diagonalnode edges are incident on northeast nodes 615 and southwest nodes 620.FIG. 7 illustrates that −45° diagonal node edges exist between northeastnode 615 and southwest nodes 620 of different Gcells. As shown in thisfigure, no −45° diagonal node edges are incident on northwest nodes 605and southeast nodes 610.

[0039] In the embodiments described below, each Manhattan node edge onlayer 2 or 3 has a unit length cost (L). In these embodiments, eachdiagonal node edge on layer 4 or 5 has a length cost that equals theunit length cost times the square root of two (L*{square root}{squareroot over (2)}). Also, the use of a node edge across a Gcell boundaryreduces the capacity of the boundary, and is thereby assessed a wirecongestion cost.

[0040] The router examines wire congestion at Gcell boundaries on eachlayer available for routing. Specifically, on each available-routinglayer, the router computes capacities at Gcell boundaries for wiringalong the particular layer's direction. On a particular layer, thewiring resources (i.e., wiring tracks) across a Gcell boundary can beconceptually represented as a planar “congestion edge” across thatboundary on the particular layer in the layer's wiring direction.

[0041]FIG. 8 presents a two-dimensional diagram that illustrates thecongestion edges on layers 2-5 for the routing directions illustrated inFIGS. 4-7. FIG. 8 illustrates one horizontal congestion edge across eachvertical boundary between horizontally adjacent Gcells, one verticalcongestion edge across each horizontal boundary between verticallyadjacent Gcells, and two 45° diagonal congestion edges across eachboundary between each pair of adjacent Gcells. In this example, eachvertical congestion edge is on layer 2, each horizontal congestion edgeis on layer 3, each 45° congestion edge is on layer 4, and each −45°congestion edge is on layer 5.

[0042] The router keeps track of one congestion-grid capacity on eachlayer at each boundary between adjacent Gcells. Accordingly, eachcongestion edge is associated with all node edges that cross the sameGcell boundary on the same layer as the congestion edge. As illustratedin FIGS. 4-7, certain planar node edges cross the Gcell boundaries. Inthe embodiments described below, certain non-planar edges between layers4 and 5 cross Gcell boundaries. These non-planar edges are furtherdescribed in Section I.B.3.

[0043] In some embodiments that use the wiring model illustrated inFIGS. 4-7, the association between the congestion edges and the nodeedges is as follows. Each horizontal congestion edge on layer 3 isassociated with the pair of horizontal node edges that cross the sameGcell boundary as the horizontal congestion edge on the layer 3. Eachvertical congestion edge on layer 2 is associated with the pair ofvertical node edges that cross the same Gcell boundary as the verticalcongestion edge on layer 2.

[0044] Each 45° diagonal congestion edge on layer 4 (1) is associatedwith a 45° diagonal node edge that crosses the same Gcell boundary asthe 45° diagonal congestion edge on layer 4, and (2) can be associatedwith two non-planar node edges between layers 4 and 5 that cross thesame Gcell boundary as the 45° congestion edge. Each −45° diagonalcongestion edge on layer 5 (1) is associated with a −45° diagonal nodeedge that crosses the same Gcell boundary as the −45° diagonalcongestion edge on layer 5, and (2) can be associated with twonon-planar node edges between layers 4 and 5 that cross the same Gcellboundary as the −45° congestion edge. The association between ±45°congestion edges and non-planar node edges will be described below inSection I.B.3.

[0045] Node edges start and terminate on nodes. Congestion edges, on theother hand, do not have explicit start and end points in someembodiments. This is because unlike node edges that are used to defineroutes, congestion edges function only to evaluate usage versuscapacity. The router's use of node and congestion edges is furtherdescribed below.

[0046] B. Non-Planar Edges: Vias.

[0047] In the embodiments described below, the router can define routesthat use non-planar node edges. In these embodiments, non-planar nodeedges exist (1) between each pair of nodes that are overlapping and thatare in two adjacent routing layers (e.g., are in layers 2 and 3), (2)between certain pairs of non-overlapping nodes that are within the sameGcell and that are on adjacent diagonal layers 4 and 5, and (3) betweencertain pairs of non-overlapping nodes that are within adjacent Gcellsand that are on adjacent diagonal layers 4 and 5. Each non-planar nodeedge represents a via between the two layers traversed by the edge. Anon-planar edge that is between non-overlapping nodes in layers 4 and 5also represents wiring to and from the edge's via. Each of thenon-planar edge types will now be described further.

[0048] 1. Non-Planar Edge Between Overlapping Nodes.

[0049] The routing graph includes a non-planar node edge between eachpair of overlapping nodes that are on two adjacent routing layers. Eachsuch non-planar edge represents a via between the edge's two nodes. Eachsuch edge is assessed a wirelength cost and a via congestion cost. Thewirelength cost equals a via-scalar factor (X) times the unit lengthcost (L) (i.e., is assessed a wirelength cost X*L). The via-scalarfactor is 1 in some embodiments, while it is greater or less than one inother embodiments. The use of any non-planar edge also incurs a viacongestion cost that represents the potential difficulty in placing toomany vias between the two layers traversed by the non-planar edge in theGcell associated with the non-planar edge's via. For a non-planar edgebetween two overlapping nodes, the Gcell associated with the edge's viasis the Gcell containing the two nodes.

[0050] 2. Non-Planar Edges Between Non-Overlapping Nodes in the SameGcell: Internal Zigs

[0051] Non-planar node edges exist between certain pairs ofnon-overlapping nodes that are within the same Gcell and that are onadjacent diagonal layers 4 and 5. Such non-overlapping nodes are calledinternal zigs. FIGS. 9 through 12 illustrate four internal zigs thatsome embodiments define between layers 4 and 5 in a Gcell. Each of thesefigures presents a two-dimensional top view of the routing graph. FIG.13 presents a three-dimensional side view of the example illustrated inFIG. 9.

[0052] In FIGS. 9 and 13, an internal zig 900 goes from a northwest node905 on layer 4 to a northeast node 910 on layer 5 in a Gcell 920. Onlayer 4, 45° node edges run through northwest nodes (such as node 905)but not through northeast nodes (such as node 910). Conversely, on layer5, −45° node edges run through northeast nodes (such as node 910) butnot through northwest nodes (such as node 905). Accordingly, theinternal zig 900 allows a route running through node 905 or 910 tochange layers and directions. As shown in FIGS. 9 and 13, this zig hasthree components. Two of its components are planar segments, where onesegment is a 45° edge that runs northerly from the sub-regionrepresented by node 905 on layer 4, while the other segment is a −45°edge that runs southerly to the sub-region represented by node 910 onlayer 5. The third component is a non-planar component that is at thelocation where the two planar components overlap. The non-planarcomponent represents a via, while the planar components represent wiringto and from the via. FIGS. 9 and 13 show the location of thisintersection (i.e., the via location) to be on the Gcell boundary.However, in a detailed route representation of the internal zig 900,this intersection might occur anywhere within the sub-region 935illustrated in FIG. 9.

[0053] The embodiments described below assess three costs for theinternal zig 900. First, an internal zig is assessed a wirelength costthat equals a via-scalar factor (X) times the unit length cost (L)(i.e., is assessed a wirelength cost X*L). Second, an internal zig isassessed an additional wirelength cost, which is the unit length costtimes the square root of two (i.e., it is L*{square root}{square rootover (2)}). This additional wirelength cost represents the approximatewirelength necessary to traverse to and from the actual via location.Third, there is a via congestion cost associated with the internal zig.This via congestion cost represents the potential difficulty in placingtoo many vias between the two layers traversed by the internal zig inthe Gcell associated with this zig's via. The Gcell associated with aninternal zig's via is the Gcell containing the two nodes of the zig.

[0054] As mentioned above, the internal zig might not result in a via inGcell 920 but might result in a via in Gcell 925 above it. Accordingly,unlike the embodiments described below, other embodiments might assess avia congestion cost for the Gcell 925 and/or assess a wire congestioncost to account for the congestion that the wiring associated with theinternal zig might cause across the boundary between Gcells 920 and 925.As further described below for external zigs, via and wire congestioncosts should be accounted for together, as the location of the via willdetermine the layer on which wires cross the congestion grid. Viacongestion and wire congestion costs are further described below.

[0055] The internal zigs 1000, 1100, and 1200 that are illustrated inFIGS. 10, 11, and 12 are analogous to the internal zig 900, except thatthey connect different pairs of nodes in the Gcell 920. These three zigsare costed in the same manner as the zig 900.

[0056] 3. Non-Planar Edges Between Non-Overlapping Nodes in the AdjacentGcells: External Zigs

[0057] Non-planar node edges exist between certain pairs ofnon-overlapping nodes that are within adjacent Gcells and that are onadjacent diagonal layers 4 and 5. Such non-overlapping nodes are calledexternal zigs. FIGS. 14 through 21 illustrate eight external zigs thatsome embodiments define between one of four nodes of a particular Gcell(920) and one of eight nodes in the four Gcells (1410, 1415, 1420, and1425) that are adjacent to the particular Gcell (920). Each of thesefigures presents a two-dimensional top view of the routing graph. FIG.22 presents a three-dimensional side view of the example illustrated inFIG. 14.

[0058]FIGS. 14 and 22 illustrate an external zig 1400 between the node930 on layer 4 of the Gcell 920 and the node 1405 on layer 5 of theGcell 1410, which is adjacent to Gcell 920. On layer 4, 45° node edgesrun through southeast nodes (such as node 930) but not through southwestnodes (such as node 1405). Conversely, on layer 5, −45° node edges runthrough southwest nodes (such as node 1405) but not through southeastnodes (such as node 930). Accordingly, the external zig 1400 allows aroute running through node 930 or 1405 to change layers and directions.

[0059] This zig traverses has three different components. Two of itscomponents are planar segments, where one segment is a 45° edge thatruns northerly from the sub-region represented by node 930 on layer 4,while the other segment is a −45° edge that runs southerly to thesub-region represented by node 1405 on layer 5. The third component is anon-planar component that is at the location where the two planarcomponents overlap. The non-planar component represents a via, while theplanar components represent wiring to and from the via.

[0060] There are four costs associated with the external zig 1400.First, an external zig is assessed a wirelength cost that equals avia-scalar factor (X) times the unit length cost (L) (i.e., is assesseda wirelength cost X*L). Second, the external zig is assessed anadditional wirelength cost, which is the unit length cost times squareroot of two (i.e., it is L*{square root}{square root over (2)}). Thisextra wirelength cost represents the approximate wirelength necessary totraverse to and from the actual via location.

[0061] The third and fourth cost components are the via congestion costand the wire congestion cost. As mentioned above, the use of anynon-planar edge incurs a via congestion cost that represents thepotential difficulty in placing too many vias between the two layerstraversed by the non-planar edge in the Gcell associated with thenon-planar edge's via. The wire congestion cost, on the other hand,represents the congestion that the wiring associated with the externalzig causes across the Gcell boundary crossed by the external zig.

[0062] The via and wire congestion costs of an external zig depend onthe actual location of the via represented by the external zig. However,an external zig specifies only that a via between layers 4 and 5 isplaced close to the boundary between two Gcells (e.g., Gcells 920 and1410), and does not specify an actual location of the via between layers4 and 5. In other words, an external zig can be associated with a vialocation in either of the two Gcells that it traverses, and can beassociated with either of the two diagonal congestion edges that aredefined across the boundary between the two Gcells. Consequently, insome embodiments, the router associates the external zig with one of theGcells and one of the diagonal congestion edges, in order to assign thevia and wire congestion costs for using the external zig. To do this,the router first computes two sets of via and wire congestion costs,where (1) the first set is based on a via location in one Gcell and on aGcell boundary-crossing along a particular congestion edge, and (2) thesecond set is based on a via location in the other Gcell and on aGcell-boundary crossing along the other congestion edge. The router thenidentifies the set with the smaller aggregate via and wire congestioncosts. It then specifies the external zig's via location and congestionedge as the identified set's via location and congestion edge.

[0063] For instance, in FIGS. 14 and 22, the via for the external zig1400 can be located in Gcell 920 or Gcell 1410. Accordingly, a first setof via and wire congestion costs V1 and W1 is computed based on anassumption that a detail route generated from the external zig 1400would result in a via location in Gcell 920. FIG. 23 illustrates such avia location. For such a location, the via congestion cost V1 iscomputed. The cost V1 represents the increase in the via congestionbetween layers 4 and 5 in the Gcell 920. Section III will describe howvia congestion costs are computed in some embodiments.

[0064] As shown in FIG. 23, the via location in Gcell 920 will require a−45° edge to cross the congestion-grid boundary 1430 between Gcells 920and 1410 on layer 5. Hence, for this via location, the wire congestioncost W1 is computed. The cost W1 represents the increase in thecongestion in the −45° direction on layer 5 across the Gcell boundary1430. This wire congestion cost is computed by reference to the capacityand usage of congestion edge 2305, which represents the wiring tracks inthe −45° direction across the Gcell boundary 1430. Section III willdescribe how wire congestion costs are computed in some embodiments.

[0065] For the external zig 1400, FIG. 24 illustrates a via location inGcell 1410. A second set of via and wire congestion costs V2 and W2 iscomputed for this via location. The via congestion cost V2 representsthe increase in the via congestion between layers 4 and 5 in the Gcell1410. Also, as shown in FIG. 24, this via location will require a 45°edge to cross the boundary 1430 on layer 4. Hence, for this vialocation, the wire congestion cost W2 represents the increase incongestion cost in the 45° direction on layer 4 across the Gcellboundary 1430. This wire congestion cost is computed by reference to thecapacity and usage of congestion edge 2310, which represents the wiringresources in the 45° direction across the Gcell boundary 1430.

[0066] Once the two sets of costs are computed for the example in FIGS.14 and 22, two aggregate values A1 and A2 are obtained by using a linearequation to sum the via and wire costs in each set. For instance, insome embodiments, A1 equals a*V1+b*W1, while A2 equals a*V2+b*W2, wherea and b are scalar values. The scalar values a and b are equal to 1 insome embodiments, while, in other embodiments, they differ from eachother and/or are greater or less than 1.

[0067] After computing the aggregate values, the external zig isassociated with the set that results in the smaller aggregate value. Forinstance, if the aggregate value A1 of the first set is smaller than thesecond set's aggregate value A2 in the example illustrated in FIGS. 23and 24, the via location, edge crossing, and via and wire congestioncosts of the first set are selected as the via location, edge crossing,and via and wire congestion costs of the external zig 1400. In otherwords, the router specifies the Gcell 920 as the Gcell that contains thevia of the external zig 1400. It associates this external zig with thecongestion edge 2305 (i.e., with a −45° edge crossing on layer 5). Therouter also specifies this external zig's incremental via and wirecongestion costs as the values V1 and W1 (i.e., as the first-setincremental via and wire congestion costs).

[0068] The via and wire congestion costs in and across Gcells are valuesthat continually evolve as the router embeds more routes. Hence, eachtime the router explores using a non-planar edge, the router uses theabove-described approach to select the optimal via location and edgecrossing for an external zig at that time.

[0069] The external zigs 1500-2100 that are illustrated in FIGS. 15-21are analogous to the external zig 1400, except that they connectdifferent node pairs. These seven external zigs 1500-2100 are costed inexactly the same manner as the zig 1400.

[0070] C. Route Representation with Respect to the Length and CongestionEdges

[0071] As described below, the router identifies the global route for anet by performing one or more path searches that identify one or moreroute segments that connect one or more pairs of pins/Steiner points ofthe net. Each path search tries to identify a path between two sets ofnodes associated with the net along the node edges. If the path searchidentifies a path between the two sets, it embeds the identified path byreference to the node edges that the path traversed to go from one nodeset to the other. Hence, the router ends up defining each net's route interms of the node edges.

[0072] However, in several instances, the discussion below refers tocongestion edges used by a route or a path, where a path is an actual orpotential portion of a route. A route or path is said to use aparticular congestion edge if it is defined by reference to a planar ornon-planar node edge that crosses the same boundary as the congestionedge on the same layer as the congestion edge. In other words, a routeor path is said to use a particular congestion edge when it uses (1) aplanar node edge associated with the particular congestion edge, or (2)a non-planar node edge that the router has associated with theparticular congestion edge for the route's or path's use of thenon-planar node edge.

[0073] Even though the embodiments described below define global routesby reference to the node edges, one of ordinary skill will realize thatother embodiments might define a global route differently. For instance,some embodiments might define a global route in terms of the congestionedges.

[0074] II. Overall Flow of Router

[0075]FIG. 25 illustrates a process 2500 that conceptually representsthe overall flow of the router in some embodiments of the invention. Asshown in this figure, the process 2500 initially uses (at 2505) thecongestion and length grids 100 and 200 to partition the IC layoutregion into numerous Gcells, with four nodes on each routing layer ineach Gcell. As described above, these Gcells and nodes define a routinggraph in which the router defines and embeds routes.

[0076] Next, the process computes (at 2510) the capacities of congestionedges between adjacent Gcells. These edges were described above byreference to FIG. 8. The capacity of a congestion edge is typicallydetermined by a variety of factors, such as the size of the sub-regions,the pitch (width and spacing) of the wiring tracks represented by theedge, and the obstructions near edge.

[0077]FIGS. 26A and 26B present two examples that illustrate how theprocess 2500 computes the capacity of a congestion edge in someembodiments. In the description of these examples, “pitch” refers to thesum of the default wire width and spacing for a given layer, projectedin the wiring direction for that layer onto a Gcell boundary that itcrosses. For a 145-degree wire, this projection increases width andspacing by a factor of {square root}{square root over (2)} compared totheir ordinary values.

[0078] To compute the capacity of a congestion edge that crosses a Gcellboundary on a given layer, the process 2500 (1) defines a parallelogramabout the Gcell boundary, (2) identifies potential obstacles in theparallelogram, (3) identifies the intersection of the projection in thelayer's wiring direction of the identified obstacle with the boundary,(4) specifies any identified intersection as a blocked portion of theboundary, and (5) derives the congestion-edge capacity from theunblocked portion of the boundary.

[0079] In some embodiments, the process identifies a parallelogram abouta Gcell boundary in the following manner. It identifies a first pair ofparallel sides of the parallelogram by translating the Gcell boundaryonto the midpoint of each of the two Gcells in the direction of thelayer's wiring direction. These two sides will be parallel to theboundary between the Gcells and will traverse through the Gcellmidpoints. The second pair of the parallelogram's parallel sides are inthe layer's wiring direction and connect to the parallelogram's firstpair of sides (i.e., each side in the second pair terminates at one endof each side of the first pair).

[0080]FIG. 26A illustrates a parallelogram 2615 that is defined for acongestion edge 2632 that crosses a Gcell boundary 2630 on layer 4,while FIG. 26B illustrates a parallelogram 2665 that is defined for acongestion edge 2634 that crosses a Gcell boundary 2680 on layer 3. Theboundary 2630 is between Gcells 2605 and 2610, while the boundary 2680is between Gcells 2655 and 2660. The diagonal congestion edge 2632 isassociated with one 45° node edge that crosses the boundary 2630 onlayer 4, while the horizontal congestion edge is associated with twohorizontal node edges that cross the boundary 2680 on layer 3.

[0081] In FIG. 26A, the wiring direction on layer 4 is the 45° diagonaldirection. Accordingly, the boundary 2630 is translated onto the centerof the Gcells 2605 and 2610 in the 45° diagonal direction. Thistranslation defines two parallel vertical sides 2620 and 2625 of theparallelogram 2615. These two sides are parallel to the boundary 2630and respectively run through the center of Gcells 2605 and 2610. Theother two sides of the parallelogram are sides 2635 and 2640, which arein the layer's wiring direction (which is the 45° direction) and connectto sides 2620 and 2625.

[0082] In FIG. 26B, the wiring direction is horizontal. Accordingly, theboundary 2680 is translated onto the center of the Gcells 2655 and 2660in the horizontal direction. This translation defined two parallelvertical sides 2670 and 2675 of the parallelogram 2665. These two sidesare parallel to the boundary 2680 and respectively run through thecenter of Gcells 2655 and 2660. The other two sides of the parallelogram2665 are sides 2685 and 2690, which are in the layer's wiring direction(which is the horizontal direction) and connect to the sides 2670 and2675.

[0083] After identifying the parallelogram about a congestion-edge'sGcell boundary, the process then identifies each potential obstacle(e.g., each piece of pin, obstruction, or pre-route metal) that falls inthe parallelogram. For each potential obstacle identified in theparallelogram, the process then identifies the portion of the boundarythat the obstacle would intersect if the obstacle were moved across theboundary in the layer's wiring direction. For instance, FIG. 26Aillustrates a pin 2642 on layer 4 that falls within the parallelogram2615. As shown in this figure, this pin would intersect portion 2644 ofthe boundary if it were moved across the boundary 2630 in the 45°direction. FIG. 26B illustrates a pin 2692 on layer 3 that falls withinthe parallelogram 2665. As shown in this figure, the pin 2692 wouldintersect portion 2694 of the boundary 2680 if it were moved across thisboundary in the horizontal direction.

[0084] The process treats all identified intersected portions of theboundary as blocked segments of the boundary. The process then estimatesthe capacity of a congestion edge to be the total length (T_(U)) of allunblocked intervals on the congestion edge's boundary that are at leastone pitch long, divided by pitch (P), i.e., the capacity of thecongestion edge equals $\frac{T_{U}}{P}.$

[0085] Some embodiments might not treat each piece of pin, obstruction,or pre-route metal as a blockage on the interval of boundary onto whichit translates in the routing direction. Also, some embodiments mightdifferently define the region to examine near a boundary crossed by acongestion edge. For instance, some embodiments might define differentparallelograms on the diagonal layers. Instead of the parallelogram 2615in FIG. 26A, some embodiments might define a parallelogram 2652illustrated in FIG. 26C. This parallelogram 2652 has the Gcell centersand the Gcell boundary endpoints as its four vertices.

[0086] After 2510, the process computes (at 2515) the via capacitybetween each two adjacent layers in each Gcell. In a given Gcell, thevia capacity between two adjacent layers is computed as auser-adjustable constant times the maximum of all capacities of planarcongestion edges into the Gcell on either of the layers. The constant istypically less than 2. In some embodiments, it is 1.7.

[0087] After 2515, the process then identifies (at 2518) a set ofpotential Steiner points for each net that it is routing. Steiner pointsfor a net can be found by (1) representing each pin of the net as aunique (x,y) position given by the centroid of its pin geometry, (2)assigning an edge cost between any two points in the plane equal to theoctilinear distance between them, and (3) invoking a procedure given in“A fast and simple Steiner routing heuristic”, by Manjit Borah, RobertOwens, and Mary Jane Irwin, Discrete Applied Mathematics 90 (1999), pp.51-67. One manner of computing the octilinear distance between twopoints is described in U.S. patent application Ser. No. 10/174,662,entitled “Method and Apparatus for Estimating Distances in a Region,”and filed on Jun. 19, 1902. The U.S. patent application Ser. No.10/174,662 is incorporated herein by reference. One skilled in the artwill recognize that a variety of other heuristics may be used togenerate Steiner points. Also, the Steiner set for a net might be anempty set in certain situations.

[0088] Next, for each net that it is being routed, the processidentifies (at 2520) a congestion-unaware route that does not accountfor via congestion within the Gcells or wire congestion at Gcellboundaries. The generation of a congestion-unaware route for a net willbe further described below in Section III.

[0089] After 2520, the process performs two nested loops. The inner loopidentifies one set of routes for each net being routed, while the outerloop causes the inner loop to run several (e.g., 8) times to generateseveral (e.g., 8) sets of routes. The generated sets of routes typicallydiffer. These sets often differ because, in the embodiments describedbelow, the inner loop uses a route-generation process that employs acosting function that accounts for resources used by the routespreviously identified by the inner loop. The outer loop runs from 2525to 2545, while the inner loop runs from 2530 to 2540.

[0090] At 2525, the process sorts the nets. In some embodiments, thefirst time the process reaches 2525 it sorts the nets in an ascendingorder of the lengths of their congestion-unaware routes, which wereidentified at 2520. The process then selects (at 2530) a net accordingto the order specified at 2525. It then identifies (at 2535) a route forthe selected net. To identify this route, the process typically uses aroute generation process that employs a costing function that accountsfor resources used by the routes previously identified at 2535. Nopreviously identified route exists for the first net in the first passof the process 2500 through 2535. However, one or more such routes existin every subsequent pass through 2535. The route identification at 2535will be further described below in Section III. As mentioned above, theprocess 2500 typically uses a route generation process at 2535 toidentify a route for a net. However, in some cases, the process mightnot identify a new route at 2535 for a net, but rather might identify aprevious route (e.g., the most recent route) that it previouslyidentified for the net.

[0091] After identifying a route for the selected net, the processdetermines (at 2540) whether it has generated a route for all the netsin the current pass through 2525-2545 (i.e., whether the selected net isthe last net in the order specified in the last pass through 2525). Ifnot, the process selects (at 2530) the next net in the order specifiedin the last pass through 2525, identifies (at 2535) a route for thisnet, and then determines (at 2540) whether this net is the last net inthe order specified in the last pass through 2525.

[0092] Once the process determines (at 2540) that it has generated aroute for all the nets in its current pass through 2525-2545, theprocess determines (at 2545) whether it has generated the desired number(S) of route sets. If not, the process returns to 2525 to initiateanother pass through the outer loop (i.e., through 2525 to 2545) so thatit can generate another set of routes. For this pass through, theprocess can specify (at 2525) the same net order as, or a different netorder than, the previous pass through the outer loop. Some embodimentsspecify a different net order for each pass through the outer loop in anattempt to increase the differences between the sets of generatedroutes.

[0093] When the process determines (at 2545) that it has generated thedesired number of route sets, the process then identifies (at 2550) oneset of routes from all the generated routes. Different embodiments usedifferent techniques to select (at 2550) one combination of routes fromthe set of identified routes. One suitable technique is randomizedrounding, which is described in Randomized Algorithm, by Rajeev Motwaniand Prabhakar Raghavan, Cambridge University Press (1995, 1997).

[0094] Several other suitable techniques are described in United Statespatent application entitled “Method and Apparatus for Solving anOptimization Problem,” filed concurrently with the present application,and filed with Express Mail Number EV169571637US. This application isincorporated herein by reference. One technique described in thisincorporated application identifies one set of routes by firstspecifying a set that has one identified route for each net. It theniteratively examines all the nets. During the examination of eachparticular net, the process iteratively examines all the identifiedroutes for the particular net. During the examination of each particularroute for each particular net, the process replaces the current routefor the particular net in the solution set with the particular route ifthe replacement would improve the solution set. Under this approach, theset that remains after all the identified routes of all the nets havebeen examined is the set identified at 2550.

[0095] After 2550, the process ends.

[0096] III. Route Generation

[0097]FIG. 27 illustrates a route-generation process 2700 that therouting process 2500 can use (at 2535) to generate a route for aparticular net in some embodiments. The process 2700 starts (at 2705) byinitializing a variable, Route_Length, to zero. The process uses thisvariable to specify the length of the route that it tries to constructfor the net.

[0098] The process then identifies (at 2710) the nodes (i.e., thelength-grid sub-regions on each layer) that contain the particular net'sset of pins and Steiner points. These nodes will be referred to as theconfiguration nodes of the particular net. After identifying theconfiguration nodes once for a net, some embodiments store theconfiguration nodes for the net, so that they can be retrieved the nexttime that they are needed. In some cases, a pin or a Steiner point canbe in more than one length-grid sub-region (i.e., more than one node).Hence, each pin or Steiner point is associated with a set of nodes.Also, in some embodiments, each Steiner point is specified only by an x-and y-coordinate. Hence, it can be on any layer in the routing graph.Accordingly, in some embodiments, the node on each layer that includesthe x- and y-coordinates of a net's Steiner point is added to the net'sconfiguration nodes.

[0099] After identifying the configuration nodes for the particular net,the process specifies (at 2715) source and target node sets for a firstpath search. In some embodiments, the process specifies the target setas a node set that is associated with a particular pin of the net. Itthen specifies the nodes of all other pins and Steiner points in thenet's configuration that are within a certain distance of the target setas source nodes. In some embodiments, this distance is a certainpercentage greater than the distance between the target set and the nodein the net's configuration closest to the target set.

[0100] On layers 4 and 5, the net pins might be in nodes that do nothave planar edges running through them. However, such nodes are targetand source nodes that need to be expanded to and from during a pathsearch. It would be inefficient to reach these nodes only throughnon-planar edges. Accordingly, to address this situation, someembodiments specify one or more nodes that are adjacent to such nodes onthe same layer as “shadow nodes.” A shadow node of a particular node ineffect augments the representation of the particular node's pin in apath search. A particular node's shadow is a source node when theparticular node is a source node, and is a target node when theparticular node is a target node. In other words, a path can expand froma particular node's shadow node when the particular node serves as asource node. When the particular node is a target of a path search, anexpansion to the particular node's shadow node is treated as anexpansion to the target node.

[0101]FIG. 28 presents one example of shadow nodes. This figureillustrates a pin 2805 in a node 2810 on layer 4. Layer 4 has 45° nodeedges that connect the northwest and southeast nodes on this layer. Node2810, however, is a northeast node that does not have a planar node edgerunning through it. This node could be a target or source node. However,it would be difficult to reach this node since it has no incident planarnode edge. Consequently, to address this situation, some embodimentsspecify node 2815 and/or node 2820 as shadow nodes of node 2805. Asshadow nodes, node 2815 and 2820 can be treated as source nodes of apath search when node 2810 is a source node, and they can be treated astarget nodes of a path search when node 2810 is a target node.

[0102] Some embodiments would specify both node 2815 and node 2820 asshadow nodes of node 2810. Other embodiments would specify only one ofthese two nodes as a shadow node. For instance, some of theseembodiments would specify node 2815 as the shadow node since pin 2805 iscloser to the node edge 2825 running through node 2815 than the nodeedge 2830 running through node 2820. Other embodiments might take thisapproach (i.e., might expand only from the shadow node that is closestto the actual pin) only when one or more node edges connected to theshadow node is not blocked (e.g., by an obstacle). When a node edgeconnecting to the shadow node that is closest to the actual pin isblocked, some of these embodiments might select the other adjacent nodeas an additional shadow node.

[0103] After specifying (at 2715) the source and target sets for a pathsearch, the process directs (at 2720) a path-generation process toidentify and embed the lowest-cost path between the specified source andtarget sets. If the path-search process embeds the lowest-cost path, thepath-generation process increments the Route_Length by the length of theembedded path. The path-generation process is further described below byreference to FIG. 29.

[0104] At 2725, the route-generation process determines whether thepath-generation process was able to identify and embed a path betweenthe specified source and target sets. If not, the process 2700 hasfailed to find a route for the net. Accordingly, it returns (at 2730) anotification specifying its failure and then ends.

[0105] The router's response to this notification was not illustrated inFIG. 25, in order not to obscure the description of the router's flowwith unnecessary details. However, it should be noted that the routerresponds differently to this notification in different embodiments. Forinstance, in some embodiments, the router can remove from the routingproblem a net that the process 2700 fails to route. In otherembodiments, the router removes the net only if it repeatedly fails tofind a route for the net after re-adjusting the net order and trying tofind a complete routing solution.

[0106] If the process determines (at 2725) that the path-generationprocess identified and embedded a path, it determines (at 2735) whetherit has routed all the pins of the net. If so, the process 2700 notifies(at 2740) the process 2700 that it has embedded a route for the net andprovides this route and its associated Route Length.

[0107] If the process 2700 determines (at 2735) that it has not routedall the pins of the net, the process specifies (at 2745) new source andtarget sets for another path search. In some embodiments, the processspecifies (at 2745) as the target node set (1) all the nodes that areassociated with the routed pins and Steiner points, and (2) all nodesthat are currently on the one or more paths that the path-generationprocess has embedded for the net during the current route generation. Insome embodiments, the process specifies (at 2745) as the source node setall nodes associated with any unrouted pin and Steiner point in thenet's configuration that are within a certain distance of the targetset. In some embodiments, this distance is a certain percentage greaterthan the distance between the target set and a node in the net'sconfiguration that is closest to the target set and that is associatedwith a pin or Steiner that has not yet been routed. After specifying thesource and target sets at 2745, the process 2700 returns to 2720 todirect the path-generation process to identify and embed the lowest-costpath between the specified source and target sets. The operation of theprocess 2700 from 2720 was described above.

[0108] A. Path Generation.

[0109] At 2720, the route-generation process 2700 calls apath-generation process to identify and embed a path between source andtarget node sets in the routing graph. In some embodiments, the routeruses an A* path-generation process 2900 that is illustrated in FIG. 29.This process has two phases: (1) a path exploration phase, during whichthe process identifies a path between the specified source and targetnode sets, and (2) a path-embedding phase, during which the processembeds the identified path.

[0110] The process 2900 is an iterative best-first search that at eachiteration tries to extend a partial solution with the best estimatedcost. Specifically, during its path exploration phase, the process 2900starts its path search by specifying the start of one or more paths fromone or more source nodes. It then iteratively identifies one or morepath expansions about the lowest cost path, until it identifies a paththat connects a source node and a target node. Each identified expansionabout a path is from a “current node” (also called “start node”) reachedby the path being extended to a “destination node” that neighbors thecurrent node.

[0111] For each expansion, the process computes an {circumflex over (F)}cost, which is the estimated cost of the path from a source node throughthe expansion's destination node to a target node. This cost can beexpressed as:

{circumflex over (F)}=G+Ĥ.  (1)

[0112] In this equation, G represents the cost of the path that hasreached the expansion's destination node, while Ĥ represents anestimated cost of a path from the expansion's destination node to theset of target nodes. In the embodiments described below, the Ĥ costexpresses the lower-bound estimate of the shortest path from theexpansion's destination node to the target set. Accordingly, in theseembodiments, the {circumflex over (F)} cost expresses the estimated costof a lowest-cost path from a source node through the expansion'sdestination node to a target node. Also, in these embodiments, the G andhence the {circumflex over (F)} account for several different types ofcosts, such as a wirelength, wire congestion, and via congestion. Eachof these costs is further described below.

[0113] As shown in FIG. 29, the process 2900 initially (at 2905)identifies and sets the Ĥ of each source node that the process 2700specified for the current path search. Each node's Ĥ expresses theestimated distance between the node and the target set in the currentpath search. During each search, the process 2900 stores the Ĥ cost foreach node after computing this cost, so that it only has to compute itonce for each node reached in each search. Different embodiments computea node's Ĥ differently. Some embodiments use a rectilinear bounding boxtechnique that is used in conventional A* path searches. Otherembodiments, however, use the novel techniques described in theabove-incorporated U.S. patent application Ser. No. 10/174,662. Onetechnique described in this application identifies two bounding boxesthat each enclose the target set. One bounding box has sides that areparallel to one of the layout's coordinate axes. The other bounding hassides that are rotated by 45° with respect to the layout's coordinateaxes. This technique then identifies the distance between the node andeach of the two bounding boxes. It then identifies the node's Ĥ as thelonger of the two identified distances.

[0114] For each source node, the process 2900 also specifies (at 2905) a“drop,” which is path identifier that represents a path expansion.Specifically, a drop represents an expansion from a start node to adestination node by referring to the destination node as its node andreferring back to the drop of the start node. Drops allow the process2900 to keep track of the paths that it explores. For each drop, theprocess also stores (1) a G cost, which is the cost of a path from asource node to the drop's node through the sequence of expansions thatled to the drop, and (2) an {circumflex over (F)} cost, which is thedrop's G cost plus the Ĥ cost of the drop's node (i.e., the Ĥ cost ofthe destination node of the expansion for which the drop was specified).One of ordinary skill will realize that other embodiments might not usedrops or might implement drops differently.

[0115] At 2905, a drop that is defined for a source node refers to thesource node as its node and defines the drop's prior drop as null. Theprocess sets the G cost of each drop defined at 2905 to zero, and setsthe drop's {circumflex over (F)} cost equal to Ĥ cost of the drop'snode. At 2905, the process stores the specified drops in a storagestructure, which, in some embodiments, is a priority queue (e.g., aheap) that is ordered based on the {circumflex over (F)} costs of thedrops.

[0116] Next, at 2910, the process retrieves from the priority queue adrop with the smallest {circumflex over (F)} cost, and specifies thisdrop as the Current_Drop. The process then “closes” (at 2915) the drop'snode. A closed node is a node to which the process can no longer expandduring the path search. Consequently, the closing of node at 2915prevents the process 2900 from expanding to this node during the currentpath search.

[0117] The process then determines (at 2920) whether the Current_Drop'snode is a node in the target set for the current path search. If not,the process performs a series of operations in a loop from 2930 to 2965,in order to explore all possible expansions about the Current_Drop.Specifically, at 2930, the process identify one of the possibleexpansions about the Current_Drop. Table 1 below lists all the possibleexpansions from the Current_Drop's node for a wiring model that allowsrouting only on layers 2-5. TABLE 1 Drop's Non-Planar Layer PlanarExpansion Expansions 2^(nd) Layer If drop's node is not on the north orsouth Expansion to the boundary of the layout, two planar node directlyexpansions are possible to the two nodes above the drop's that areadjacent to the drop's node in the node. ±90° directions. If the drop'snode is on the south or north layout boundary, then one planar expansionis available to the one node adjacent to the drop's node in the +90° or−90° direction. 3^(rd) Layer If drop's node is not on the east or westExpansion to boundary of the layout, two planar nodes directlyexpansions are possible to the two nodes above and below that areadjacent to the drop's node in the the drop's node. 0° and 180°directions. If the drop's node is on the east or west boundary of thelayout, then one planar expansion is available to the one node adjacentto the drop's node in the +180° or 0° direction. 4^(th) Layer If drop'snode is not on periphery of the Expansion to layout and it is either thenorthwest or nodes directly southeast node of a Gcell, two planar aboveand below expansions are possible to the two nodes the drop's node. thatare adjacent to the drop's node in the Also, two internal +45° and −135°directions. If the drop's zig expansions, node is on the periphery ofthe layout and where each it is either the northwest or southeast nodeexpansion is to a of a Gcell, then zero planar expansion is node that isin possible or one planar expansion is layer 5 in the available to theone node adjacent to same Gcell as the the drop's node in the +45° or−135° drop's node. direction. There are no planar expansions Up to twofrom a drop's node if this node is the external zig northeast orsouthwest node of a Gcell. expansions, where each such expansion is to anode that is in layer 5 in another Gcell which is adjacent to the Gcellcontaining the drop's node. 5^(th) Layer If drop's node is not onperiphery of the Expansion to layout and it is either the southwest ornodes directly northeast node of a Gcell, two planar above and belowexpansions are possible to the two nodes the drop's node. that areadjacent to the drop's node in the Also, two internal −45° and +135°directions. If the drop's zig expansions, node is on the periphery ofthe layout and where each it is either the southwest or northeast nodeexpansion is to a of a Gcell, then zero planar expansion is node that isin possible or one planar expansion is layer 4 in the available to theone node adjacent to the same Gcell as the drop's node in the −45° or+135° drop's node. direction. There are no planar expansions Up to twofrom a drop node if this node is the south- external zig ex- east ornorthwest node of a Gcell. pansions, where each such expansion is to anode that is in layer 4 in another Gcell which is adjacent to the Gcellcontaining the drop's node.

[0118] In some embodiments that use a wiring model that allows routingon layer 1, the expansion possibilities on layer 1 are similar to theexpansion possibilities on layer 3, except that there are no non-planarexpansions to a layer below. Also, in these embodiments, the process canexpand from a node on layer 2 to a node directly below on layer 1.

[0119] After selecting an expansion at 2930, the process determines (at2935) whether the destination node of the expansion is a closed node. Ifso, the process transitions to 2965, which is further described below.Otherwise, the process computes (at 2940) a G cost for the expansion.The computation of this cost is described further below.

[0120] After 2940, the process determines (at 2945) whether thedestination node of the expansion specified at 2930 has been previouslyreached in the current path search. If not, the process (at 2960)computes and stores the destination node's Ĥ cost. The computation ofthis cost was described above at 2905. At 2960, the process alsospecifies a drop for the expansion specified at 2930. The processassociates the specified drop with the expansion's destination node, andsets the drop's previous drop to the Current_Drop. The process also (1)sets this specified drop's G cost to the G cost computed at 2940 for theexpansion, and (2) sets the specified drop's {circumflex over (F)} costto the sum of the drop's G cost and the Ĥ cost of the drop's node (i.e.,the expansion's destination node). The process then stores (at 2960) thedrop specified at 2960 in the priority queue based on its {circumflexover (F)} cost. From 2960, the process transitions to 2965, which willbe described below.

[0121] If the process determines (at 2945) that the specifiedexpansion's destination node has been previously reached in the currentpath search, the process determines (at 2950) whether the identifiedexpansion's G cost (computed at 2940) is less than the G cost of thedrop in the priority queue that is associated with the expansion'sdestination node. If not, the expansion specified at 2930 is not thecheapest expansion to its destination node. Hence, in this situation,the process stops its examination of the expansion identified at 2930,and transitions to 2965, which is further described below.

[0122] On the other hand, if the process determines (at 2950) that theidentified expansion's G cost (computed at 2940) is less than the G costof the drop in the priority queue that is associated with theexpansion's destination node, the process removes the drop associatedwith the destination node from the priority queue, and specifies a newdrop for this node. The process associates the new drop with theidentified expansion's destination node, and sets the drop's previousdrop to the Current_Drop. The process also (1) sets this newly specifieddrop's G cost equal to the identified expansion's G cost (computed at2940) for the expansion, and (2) sets the specified drop's {circumflexover (F)} cost to the sum of the drop's G cost and the Ĥ cost of thedrop's node (i.e., the Ĥ cost of the expansion's destination node). Theprocess then stores (at 2955) the newly specified drop in the priorityqueue based on its {circumflex over (F)} cost. From 2955, the processtransitions to 2965.

[0123] At 2965, the process determines whether there is any expansionabout the Current_Drop's node that it has not yet examined. If so, theprocess transitions back to 2930 to identify another expansion, and thenperforms the subsequent operations to determine whether to specify adrop for this newly identified expansion.

[0124] When the process determines (at 2965) that it has examined allexpansions about the Current_Drop's node, the process determines (at2970) whether the priority queue that stores the drops is empty. If so,the process has failed to find a path between the specified source andtarget sets. Accordingly, it returns (at 2975) a notification specifyingits failure and then ends. On the other hand, when the processdetermines (at 2970) that the priority queue is not empty, the processtransitions back to 2910 to retrieve the drop with the smallest{circumflex over (F)} cost from the priority queue and then to performthe above-described operations for this drop.

[0125] The process has found a path between the source and target setswhen it determines (at 2920) that the Current_Drop's node is a target.In this situation, the process transitions from 2920 to 2925. At 2925,the process also embeds the identified path between the source andtarget sets. Starting at the Current_Drop on the target, the embedding“back traces” the sequence of drops that reached the target andgenerates an ordered list of (1) nodes associated with the drops and (2)node edges between these nodes.

[0126]FIG. 30 illustrates an example of a back trace operation. In thisexample, the path search has identified a path between a source node3005 and a target node 3010. This path has traversed through layers 3-5.To identify this path, the path search has identified a series of drops3015 a-3015 h. With the exception of the first source-node drop 3015 athat has a null back reference, each drop has a reference to a previousdrop in the path. The back trace would start at the drop on the targetand follow each drop's back reference to identify all of the path'sdrops, the nodes associated with these drops, and the edges between thenodes of successive drops. In this manner, the back trace would identifyan ordered list of nodes 3020 between the source and target nodes 3005and 3010, and an ordered list of edges 3025 a-3025 g between thesenodes.

[0127] Some embodiments then define the global route path by referenceto the ordered list of node edges produced through the back trace. Inthe example illustrated in FIG. 30, this ordered list would includeedges 3025 a-3025 g. Other embodiments would identify the global routepath by reference to the ordered list(s) of nodes and node edgesidentified in the back trace. In the example illustrated in FIG. 30,this ordered list(s) would include edges 3025 a-3025 g and node 3005,nodes in the set 30020, and node 3010.

[0128] At 2925, the process 2900 increments the Route_Length by thelength of the identified global route path. This length is simply thesum of the length of the node edges identified through the back trace,where the length of a planar horizontal or vertical node edge is L, thelength of a diagonal node edge is L*{square root}{square root over (2)},the length of a non-planar nodes edge between overlapping nodes is X*L,and the length of a non-planar edge between non-overlapping nodes is(X+{square root}{square root over (2)})*L.

[0129] At 2925, the process also determines if the source node that itreached through its back trace at 2925, includes one or more pins thatare marked as unrouted. If the process identifies one or more such pinsat 2925, it then selects one of these pins and marks it as routed. Whenthere are no such pins in the source node reached through the backtrace, the source node contains at least one Steiner point that ismarked unrouted. Hence, when the source node does not contain anunrouted pin, the process selects (at 2925) a previously unroutedSteiner point and marks it as routed.

[0130] In some cases, the source and target node sets partially orcompletely overlap. In such cases, the path search transitions to 2925the first time it reaches 2920, and hence it will not identify anyexpansions. At 2925, the back trace then simply identifies a previouslyunrouted pin or Steiner in a node that is in both the source and targetnode sets as a routed pin or Steiner. In other words, this back traceresults in an empty set of global-route node edges as a pin or Steineris reached without requiring any node edges. In this situation, someembodiments define a special “node-internal” edge between a previouslyunrouted “point” and a newly routed “point” in the source/target node,where a point in this case can be a pin or a Steiner point. Some ofthese embodiments use such an edge purely for internal bookkeeping,while others augment the notion of the global route to include thisnode-internal edge. When all of a net's pins are in the same node, therouter only identifies node-internal edges. In such a situation, theresulting global route can be specified as either null (e.g., an emptyset of node edges), or as a collection of node-internal edges.

[0131] One of ordinary skill will realize that the path-generationprocess might be implemented differently in other embodiments. Forinstance, in some embodiments, the Ĥ cost might not specify a lowerbound on the shortest path between a drop's node and a target set. Inaddition, some embodiments might compute the {circumflex over (F)} costslightly differently. For instance, some embodiments might express the{circumflex over (F)} cost as:

{circumflex over (F)}=G+2*Ĥ.

[0132] Such a cost would bias the search process to expand about thedrops that are closer to the target set. Alternative embodiments mightexpress the {circumflex over (F)} cost as:

{circumflex over (F)}=G+Ĥ+Ĵ,

[0133] where Ĵ represents the estimated computational effort needed tocomplete the path from the current drop. The embodiments that usealternative {circumflex over (F)} cost might not satisfy theadmissibility requirement. Also, instead of an A* search, otherembodiments might perform other types of path searches.

[0134] B. Costing an Expansion

[0135] As described above, the path-generation process 2900 computes theG cost of an expansion at 2940. The embodiments described below use acost function with several exponential components that depend onwirelength, wire-congestion, and via-congestion costs. In costing anexpansion, the congestion components account for all the routes that therouter has identified at 2535 before the current path search on theedges being considered. In this manner, these components bias the pathsearch to spread the routes to favor evenly distributed congestionprofiles over those with sharp peaks. Even though exponential cost termsare described below, one of ordinary skill will realize that otherembodiments might use other types of costing functions to evaluate theroutes.

[0136] 1. General Exponential Costing Expression

[0137] For some embodiments, Equation (A) below provides a cost functionthat represents the G cost of an expansion to a destination node d.$\begin{matrix}{G = {( {Y_{L}*e^{ɛ_{L}*\frac{{\hat{F}}_{L}{(d)}}{{\hat{H}}_{MIN}}}} ) + \quad ( {\sum\limits_{j = 1}^{m}{Y_{j}*\quad ( {e^{ɛ_{C}*\frac{{new}\quad {{usage}{(j)}}}{{{goal}{({{layer}{(j)}})}}*{{capacity}{(j)}}}} - \quad e^{ɛ_{C}*\frac{{old}\quad {{usage}{(j)}}}{{{goal}{({{layer}{(j)}})}}*{{capacity}{(j)}}}}} )}} ) + \quad ( {\sum\limits_{t = 1}^{K}{Y_{t}\quad( {e^{ɛ_{V}*\frac{{new}\quad {{usage}{(t)}}}{{{goal}{({{layer}\quad {{pairs}{(t)}}})}}*{{capacity}{(t)}}}} - \quad e^{ɛ_{V}*\frac{{old}\quad {{usage}{(t)}}}{{{goal}{({{layer}\quad {{pairs}{(t)}}})}}*{{capacity}{(t)}}}}}\quad )}} }} & (A)\end{matrix}$

[0138] In this equation, the first exponential component represents awirelength cost, the second exponential component represents a wirecongestion cost, and the third exponential component represents a viacongestion cost. Table 2 provides a definition for the variables inthese components. TABLE 2 Term Definition e The base of the naturallogarithm. ε_(L), ε_(C), ε_(V) User-adjustable exponential-multiplierparameters. Y_(L), Y_(j), and Y_(t) Normalizing variables. {circumflexover (F)}_(L) (d) The estimated length of the path from a source nodethrough the expansion's destination node d to a target node. Thisestimated length equals the length of the path that has reached theexpansion's destination node d, plus the destination node's Ĥ. Ĥ_(MIN)The minimum Ĥ of the set of sources of the current path search. j One ofm congestion edges that are used by the path that has reached theexpansion's destination node d. If the path has not used any congestionedge, then m equals 0 and the wire congestion component of Equation (A)is zero. old usage(j) The cumulative number of times that all the routespreviously identified at 2535 use the congestion edge j. In someembodiments, the router computes and updates the old usage(j) valueafter each iteration of 2535 of the process 2500. new usage(j) Thecumulative number of times that the congestion edge j is used by (1) allthe routes previously identified at 2535, and (2) the path that hasreached the expansion's destination node d. New usage (j) equals oldusage (j) plus 1. capacity(j) The estimated number of available tracksof the congestion edge j. In some embodiments, this capacity is a valuefrom 0 to 16. This number was computed at 2510 of the process 2500.goal(layer(j)) A target upper bound on the congestion ratio on the layerthat contains edge j. t One of K non-planar edges that are on the paththat has reached the expansion's destination node d. If the path has notused any non-planar edge, then t equals 0 and the via congestioncomponent of Equation (A) is zero. old usage(t) In all the routespreviously identified at 2535, the number of non-planar edges that viain the same Gcell as edge t between the same two layers as edge t. Withthe exception of external zigs, the Gcell that contains the viarepresented by edge t is the Gcell that contains the two nodes connectedby edge t. If edge t is an external zig, the Gcell of the via associatedwith the external zig edge t is the Gcell that results in the smalleraggregate via and wire congestion costs for this instance of use of theexternal zig t, as described in Section I.B.3. After each iteration of2535 of the process 2500, the router in some embodiments computes andupdates the via usage values for each pair of adjacent routing layers ineach Gcell. new usage(t) In all the routes previously identified at2535, and in the path that has reached the expansion's destination noded, the number of non-planar edges that via in the same Gcell as edge tbetween the same two layers as edge t. See discussion in the definitionof old usage (t) regarding the Gcell that contains edge t's via. Newusage(t) equals old usage(t) plus 1. capacity(t) The estimated number ofvias that can traverse the same two layers as edge t in this edge'sGcell. In some embodiments, this capacity is a value from 0 to 27. Thiscapacity was computed at 2515 of the process 2500. goal(layer pairs(t))A target upper bound on the via congestion ratio between the twoadjacent layers traversed b the non-planar edge t.

[0139] Each of the three components in Equation (A) includes anexponential expression. The exponential expressions are normalized tothe same scale by multiplying them by the normalizing factors Y_(L),Y_(j), and Y_(t). In some embodiments, the multiplier Y_(L) is expressedby the following equation:${Y_{L} = {Y_{LI}*e^{ɛ_{L}*\frac{\sum\limits_{N = 1}^{R}{{Length}{(N)}}}{T.E.L}}}},$

[0140] where (1) Y_(LI) is a constant, (2) N is one of the R routesidentified thus far at 2535, (3) length (N) is the length of the routeN, and (4) T.E.L stands for total estimated length and equals the sum ofthe estimated length of each net's route. In some embodiments, the totalestimated length is the sum of the lengths of the congestion-unawareroutes (identified at 2520) of all nets; in other embodiments, it is thesum of a lower_bound on the route lengths of all the nets (e.g., it isthe sum of the bounding box of each net).

[0141] In some embodiments, Y_(LI) equals 1, while it equals anothervalue (such as$( {{such}\quad {as}\quad \frac{1}{T.E.L.}} )$

[0142] in other embodiments. The multiplier Y_(L) is a value that isre-computed after the identification of each route at 2535. Hence, eachtime that the router is identifying (at 2535) a route for a net, themultiplier Y_(L) is based on all the routes that the router hasidentified before this iteration of 2535.

[0143] The multiplier Y_(j) is also different in different embodiments.For instance, in some embodiments, this multiplier equals 1 or someother constant. Some embodiments might use different constants fordifferent edges. In other embodiments, this multiplier equals$\frac{1}{{goal}( {( {{layer}(j)} )*{{capacity}(j)}} },$

[0144] where goal (layer (j)) and capacity (j) are defined in Table 2above. In still other embodiments, this multiplier is represented by thefollowing equation: ${Y_{j} = \frac{1}{{\overset{\_}{C}}_{L}}},$

[0145] where {overscore (C)}_(L) is the average initial-capacityconstant on each layer and is represented by:${\overset{\_}{C}}_{L} = {\frac{1}{{Number}\quad {of}\quad {Layers}}{\sum\limits_{{Layers}\quad L}{( {e^{(\frac{ɛ_{C}}{{{goal}{(L)}}*{({{Average}\quad {Planar}\quad {Edge}\quad {Capacity}\quad {on}\quad L})}})} - 1} ).}}}$

[0146] Specifying the multiplier Y_(j) based on the averageinitial-capacity cost {overscore (C)}_(L) centers the initial capacitycosts of the edges about 1.

[0147] The multiplier Y_(t) is also different in different embodiments.For instance, in some embodiments, this multiplier equals to 1 or someother constant. Some embodiments might use different constants fordifferent non-planar edges. In other embodiments, this multiplier equals$\frac{1}{{goal}( {( {{layer}\quad {{pairs}(t)}} )*{{capacity}(t)}} },$

[0148] where goal (layer pairs (t)) and capacity (t) are defined inTable 2 above. In still other embodiments, this multiplier isrepresented by the following equation:${Y_{j} = \frac{1}{{\overset{\_}{C}}_{LP}}},$

[0149] where {overscore (C)}_(LP) is the average initial-capacityconstant for each layer pair and is represented by:${\overset{\_}{C}}_{LP} = {\frac{1}{{Number}\quad {of}\quad {Adjacent}\quad {Layer}\quad {Pairs}}{\sum\limits_{{Layers}\quad {Pairs}\quad {LP}}{( {e^{(\frac{ɛ_{V}}{{{goal}{({LP})}}*{({{Average}\quad {Non}\text{-}{Planar}\quad {Edge}\quad {Capacity}\quad {Between}\quad {LP}})}})} - 1} ).}}}$

[0150] Specifying the multiplier Y_(t) based on the averageinitial-capacity constant {overscore (C)}_(LP) centers the initialcapacity costs of the non-planar edges about 1. One of ordinary skillwill realize that other embodiments might specify their normalizingconstants differently.

[0151] In all three components of Equation (A), the base of theexponential expression is the natural-logarithm base e. Otherembodiments, however, might use a different base. In addition, otherembodiments might formulate differently the exponent of each exponentialexpression in Equation (A). In some embodiments, the exponentialmultipliers ε_(L), ε_(C), ε_(V) are real numbers between 2 to 16. Forinstance, in some embodiments, all three multipliers equal 9, eventhough these multipliers do not need to be the same value. As mentionedabove, each of these multipliers is adjustable by the user in someembodiments.

[0152] Each route or path includes a set of node edges, which can beplanar or non-planar. The cost of each planar and non-planar node edgewas described above in Section I. Table 3 reiterates each of these costsfor a route or a path, and describes how each of these costs is factoredin each of the components of Equation (A). TABLE 3 Wirelength Wire ViaNode Edge Component Congestion Component Congestion Component PlanarNode Increments If edge f goes from one Gcell No effect. Edge f onlength of route to another (i.e., crosses a Layers 2 and 3 or path byunit Gcell boundary), then edge f length cost L. is associated with acongestion edge j, and therefore a new usage(j) is defined byincrementing old usage(j) by one. Otherwise, no effect. Planar NodeIncrements Define new usage(j) by No effect. Edge f on length of routeincrementing old usage(j) by Layers 4 and 5 or path by one, where nodeedge f is length cost associated with congestion L * {squareroot}{square root over (2)}. edge j. Non-Planar Increments No effect.Define new usage(t) by Node Edge t length of route incrementing oldusage(t) between directly or path by (associated with vias overlappinglength cost between layers a and b in nodes on layers X*L, where X Gcellg) by one. a and b in Gcell is a via-scalar g. factor. Internal Zig tIncrements No effect. Define new usage(t) by between non- length ofroute incrementing old usage(t) overlapping or path by (associated withvias nodes on layers length cost X*L between layers a and b in a and bin Gcell plus L * {square root}{square root over (2)}. Gcell g) by one.g. External Zig t Increments Use the approach described in Define newusage(t) by between non- length of route Section I.B.3 to associate theincrement old usage(t) by overlapping or path by instance of theexternal zig t's one, where old usage(t) is nodes on layers length costX*L use with a congestion edge j. the via usage between a and b in plusL * {square root}{square root over (2)}. Define new usage(j) by layers aand b in the Gcell adjacent Gcells incrementing old usage(j) by that isassigned to contain g1 and g2. one. via for this instance of edge t'suse, per the approach described above in Section I.B.3.

[0153] One of ordinary skill will realize that other embodiments mightuse different exponential cost functions than the one illustrated inEquation (A). For instance, some embodiments might use an equation thathas the same wire and via congestion components as Equation (A), but thefollowing wirelength component instead of Equation (A)'s wirelengthcomponent.${{Wirelength}\quad {Component}} = {Y_{L1}*^{ɛ_{L}*\frac{\sum\limits_{N = 1}^{R}{{Length}\quad {(N)}}}{T.E.L.}}*{Length}\quad {(d).}}$

[0154] In this equation, Length (d) is the length of the path p that hasreached the expansion's destination node d. Other embodiments might usean equation that uses both this wirelength component and the wirelengthcomponent of Equation (A).

[0155] 2. Deriving Expansion Cost from the G Cost of the Expansion'sStart Node

[0156] Equation (A) provides a general expression of the costingfunction that represents the G cost of an expansion in some embodiment.However, in some embodiments, the process 2900 does not actually useEquation (A) to compute the G cost of an expansion. Instead, it derivesthis G cost from the G cost of the Current_Drop in the following manner.

[0157] Assume that the Current_Drop specifies (i.e., is the last dropof) a path p′, while the expansion from the Current_Drop specifies acurrent path p, which is an extension of the path p′ to the expansion'sdestination node. The Current_Drop's G cost, G(Current_Drop), is thecost of the path p′ that has reached the Current_Drop.

[0158] The process 2900 first computes a G₁ cost that is illustrated inEquation (B) below. $\begin{matrix}{G_{1} = {{G({Current\_ Drop})} + {Y_{L}*( {^{ɛ_{L}*\frac{{\hat{F}}_{L}{(p)}}{{\hat{H}}_{MIN}}} - ^{ɛ_{L}*\frac{{\hat{F}}_{L}{(p^{\prime})}}{{\hat{H}}_{MIN}}}} )}}} & (B)\end{matrix}$

[0159] In this equation, {circumflex over (F)}_(L)(p) equals the lengthof a path p plus the destination node's Ĥ. It represents an estimatedlength of a path from a source node through the expansion's destinationnode d to a target node. It is equivalent to {circumflex over(F)}_(L)(d), which was described above. {circumflex over (F)}_(L)(p′)equals the length of a path p′ plus the Ĥ of the expansion's start node.It represents an estimated length of a path from a source node throughthe expansion's start node to a target node. The remaining terms ofEquation (B) are as described above for Equation (A). The exponentialexpression$Y_{L}*^{ɛ_{L}*\frac{{\hat{F}}_{L}{(p^{\prime})}}{{\hat{H}}_{MIN}}}$

[0160] represents the wirelength cost of path p′, while the exponentialexpression $Y_{L}*^{ɛ_{L}*\frac{{\hat{F}}_{L}{(p)}}{{\hat{H}}_{MIN}}}$

[0161] represents the wirelength cost of path p. Hence, Equation (B)illustrates that the G₁ cost can be obtained by adding the incrementalwirelength cost for the expansion to the G cost of the Current_Drop,since path p is an extension of path p′ by the current expansion.

[0162] The length of the path p can be obtained from the length of thepath p′, as illustrated in Table 4 below. TABLE 4 Expansion toDestination Node Length of Path P Planar expansion on layer 2 or 3Length of p′ plus L, where L is the unit length cost. Planar expansionon layer 4 or 5 Length of p′ plus L * {square root}{square root over(2)}. Non-planar expansion between Length of p′ plus X*L, where X is aoverlapping nodes via-scalar factor. Internal or External Zigs Length ofp′ plus (X + {square root}{square root over (2)}) * L.

[0163] The G cost of the expansion equals the G₁ cost expressed inEquation (B) if the expansion is a planar expansion that does not crossa Gcell boundary. However, if the expansion is a planar expansion alonga node edge f that crosses a Gcell boundary, and the node edge f isassociated with a congestion edge j, then the expansion's G cost is a G₂cost expressed in Equation (C). $\begin{matrix}{G_{2} = {G_{1} + {Y_{j}*{( {^{ɛ_{c}*\frac{{new}\quad {usage}\quad {(j)}}{{goal}\quad {({{layer}\quad {(j)}})}*{capacity}\quad {(j)}}} - ^{ɛ_{c}*\frac{{old}\quad {usage}\quad {(j)}}{{goal}\quad {({{layer}\quad {(j)}})}*{capacity}\quad {(j)}}}} ).}}}} & (C)\end{matrix}$

[0164] As illustrated in this equation, the G₂ cost equals the G₁ costexpressed in Equation (B) plus an exponential wire congestion cost forcrossing the Gcell boundary. In Equation (C), the terms are as definedabove. The two exponential terms in Equation (C) represent costs afterand before the expansion. Hence, Equation (C) illustrates that the G₂cost can be obtained by adding the incremental wire congestion cost forthe expansion to the G₁ cost.

[0165] If the expansion is along a non-planar edge t between twodirectly overlapping nodes or is along an internal zig expansion tbetween two non-overlapping nodes in a Gcell, the expansion's G cost isa G₃ cost expressed in Equation (D) below. $\begin{matrix}{G_{3} = {G_{1} + {Y_{t}*{( {^{ɛ_{v}*\frac{{new}\quad {usage}\quad {(t)}}{{goal}\quad {({{layer}\quad {pairs}\quad {(t)}})}*{capacity}\quad {(t)}}} - ^{ɛ_{v}*\frac{{old}\quad {usage}\quad {(t)}}{{goal}\quad {({{layer}\quad {pairs}\quad {(t)}})}*{capacity}\quad {(t)}}}} ).}}}} & (D)\end{matrix}$

[0166] As illustrated in this equation, the G₃ cost equals the G₁ costexpressed in Equation (B) plus exponential via congestion cost due tothe via expansion. In Equation (D), the terms are as defined above. Thetwo exponential terms in Equation (D) represent costs after and beforethe expansion. Hence, Equation (D) illustrates that the G₃ cost can beobtained by adding the incremental via congestion cost for the expansionto the G₁ cost.

[0167] If the expansion is an external zig t that connects twonon-overlapping nodes in two adjacent Gcells, the expansion's G cost isa G₄ cost expressed in Equation (E) below. $\begin{matrix}{G_{4} = {G_{1} + {Y_{j}*( {^{ɛ_{c}*\frac{{new}\quad {usage}\quad {(j)}}{{goal}\quad {({{layer}\quad {(j)}})}*{capacity}\quad {(j)}}} - ^{ɛ_{c}*\frac{{old}\quad {usage}\quad {(j)}}{{goal}\quad {({{layer}\quad {(j)}})}*{capacity}\quad {(j)}}}} )} + {Y_{t}*{( {^{ɛ_{v}*\frac{{new}\quad {usage}\quad {(t)}}{{goal}\quad {({{layer}\quad {pairs}\quad {(t)}})}*{capacity}\quad {(t)}}}*^{ɛ_{v}*\frac{{old}\quad {usage}\quad {(t)}}{{goal}\quad {({{layer}\quad {pairs}\quad {(t)}})}*{capacity}\quad {(t)}}}} ).}}}} & (E)\end{matrix}$

[0168] As illustrated in this equation, the G₄ cost equals the G₁ costexpressed in Equation (B) plus exponential via and wire congestion costsfor the via expansion. In Equation (E), the terms are as describedabove. The two positive exponential terms represent costs after theexpansion, while the two negative exponential terms represent costsbefore the expansion. Hence, Equation (E) illustrates that the G₄ costcan be obtained by adding the incremental wire and via congestion costsfor the expansion to the G₁ cost.

[0169] To compute the incremental via and wire congestion costs, thepath search process needs to associate this instance of the external zigt's use with a via location and congestion edge, pursuant to theapproach described in Section I.B.3. Specifically, the process examinestwo different via locations and congestion edges for this use of theexternal zig t, and associates the external zig with the via locationand edge crossing that results in the smaller aggregate via and wirecongestion costs. The process then uses the incremental via and wirecongestion costs of the associated via location and edge crossing inEquation (E) to express the cost of the path p.

[0170] For instance, assume that the external zig t is zig 1400 that wasdescribed above by reference to FIGS. 14 and 22-24. Assume further thatfor this instance of external zig t, the smaller aggregate via and wirecongestion cost can be obtained by placing the external zig's via inGcell 1410. Hence, for this use of the external zig, the path searchprocess associates this use of the external zig 1400 with the congestionedge 2310, and specifies the Gcell for the external zig's via as theGcell 1410. In this situation, the incremental via congestion cost isbased on the old and new usage values for vias between layers 4 and 5 inGcell 1410. The incremental wire congestion cost is based on the old andnew usage values for the congestion edge 2310.

[0171] C. Generating a Congestion-Unaware Route for a Net

[0172] As described above, the process 2500 identifies (at 2520) thecongestion-unaware route for each net. To generate thecongestion-unaware route for a net, the process can use route-generationand path-generation processes that are similar to the above-describedroute-generation and path-generation processes 2700 and 2900, except forthe costing of expansions at 2940. To generate the congestion-unawareroute for a net, some embodiments cost the expansions at 2940 in anon-exponential manner that disregards the via and wire congestion costsand focuses solely on the wirelength cost. For instance, in theseembodiments, a planar expansion in layer 2 or 3 has a G cost that equalsthe Current_Drop's G cost plus a unit length cost L. A planar expansionin layer 4 or 5 has a G cost that equals the Current_Drop's G cost plusL*{square root}{square root over (2)}. A non-planar expansion betweentwo overlapping nodes has a G cost that equals the Current_Drop's G costplus X*L, where X is the via-scaling factor. An internal zig or anexternal zig expansion has a G cost that equals the Current_Drop's Gcost plus (X+{square root}{square root over (2)})*L. Some embodimentsdisallow expansion on a node edge for which the associated congestionedge has a capacity less than 1.

[0173] IV. Computer System

[0174]FIG. 31 conceptually illustrates a computer system with which oneembodiment of the invention is implemented. Computer system 3100includes a bus 3105, a processor 3110, a system memory 3115, a read-onlymemory 3120, a permanent storage device 3125, input devices 3130, andoutput devices 3035.

[0175] The bus 3105 collectively represents all system, peripheral, andchipset buses that support communication among internal devices of thecomputer system 3100. For instance, the bus 3105 communicativelyconnects the processor 3110 with the read-only memory 3120, the systemmemory 3115, and the permanent storage device 3125.

[0176] From these various memory units, the processor 3110 retrievesinstructions to execute and data to process in order to execute theprocesses of the invention. The read-only-memory (ROM) 3120 storesstatic data and instructions that are needed by the processor 3110 andother modules of the computer system. The permanent storage device 3125,on the other hand, is a read-and-write memory device. This device is anon-volatile memory unit that stores instruction and data even when thecomputer system 3100 is off. Some embodiments of the invention use amass-storage device (such as a magnetic or optical disk and itscorresponding disk drive) as the permanent storage device 3125. Otherembodiments use a removable storage device (such as a floppy disk orzip® disk, and its corresponding disk drive) as the permanent storagedevice.

[0177] Like the permanent storage device 3125, the system memory 3115 isa read-and-write memory device. However, unlike storage device 3125, thesystem memory is a volatile read-and-write memory, such as a randomaccess memory. The system memory stores some of the instructions anddata that the processor needs at runtime. In some embodiments, theinvention's processes are stored in the system memory 3115, thepermanent storage device 3125, and/or the read-only memory 3120.

[0178] The bus 3105 also connects to the input and output devices 3130and 3135. The input devices enable the user to communicate informationand select commands to the computer system. The input devices 3130include alphanumeric keyboards and cursor-controllers. The outputdevices 3135 display images generated by the computer system. Forinstance, these devices display IC design layouts. The output devicesinclude printers and display devices, such as cathode ray tubes (CRT) orliquid crystal displays (LCD).

[0179] Finally, as shown in FIG. 31, bus 3105 also couples computer 3100to a network 3165 through a network adapter (not shown). In this manner,the computer can be a part of a network of computers (such as a localarea network (“LAN”), a wide area network (“WAN”), or an Intranet) or anetwork of networks (such as the Internet). Any or all of the componentsof computer system 3100 may be used in conjunction with the invention.However, one of ordinary skill in the art will appreciate that any othersystem configuration may also be used in conjunction with the invention.

[0180] The above-described router can produce multi-layer global routesthat have horizontal, vertical, and diagonal edges. FIG. 32 provides anexample of one such global route. This route 3200 traverses layers 3, 4,and 5. As shown in this figure, this route includes a via between layers3 and 4, and an external zig via between layers 4 and 5.

[0181] While the invention has been described with reference to numerousspecific details, one of ordinary skill in the art will recognize thatthe invention can be embodied in other specific forms without departingfrom the spirit of the invention. For instance, even though the routerdescribed above is a flat global router, one of ordinary skill willrealize that the invention can be practiced with hierarchical routers,such as a router described in U.S. patent application Ser. No.10/013,819, filed on Dec. 7, 2001.

[0182] Also, the routes and paths described above are defined withrespect to the node edges illustrated in FIGS. 4 through 7. These routesand paths, however, can be defined differently. For instance, they canbe defined with respect to a set of edges that are orthogonal to theedges illustrated in FIGS. 4 through 7.

[0183] In addition, many aspects of the invention can be practicedwithout the congestion and length grids illustrated in FIGS. 1 and 2.Alternatively, they can be practiced with different congestion andlength grids, or different structures for these grids. For instance, insome embodiments, intersecting horizontal and vertical lines might notform one or both these grids. Also, nodes might not be defined in thecenter of the length-grid sub-regions.

[0184] Although the router described above uses horizontal, vertical,and ±45 diagonal wiring, many aspects of the invention can be practicedwith a different set of interconnect lines. Also, some embodiments mightuse costing equations that are different than those described inEquations (A)-(E) above. For instance, some embodiments described aboveexpress the wirelength cost of a path p as$Y_{L}*{^{ɛ_{L}*\frac{{\hat{F}}_{L}{(p)}}{{\hat{H}}_{MIN}}}.}$

[0185] Some embodiments might express such a path cost as${Y_{L}*^{ɛ_{L}*\frac{{\hat{F}}_{L}{(p)}}{{\hat{H}}_{MIN}}}},$

[0186] where B is a factor related to the importance of the net. Thisfactor B is smaller (e.g., it is 1) for important time-critical netsthat need shorter routes, while it is larger (e.g., it is 3) fornon-critical nets that can have longer routes. Accordingly, this factorcauses the path search to try to identify shorter paths for criticalnets, by increasing the wirelength cost of these nets much faster duringpath searches than the wirelength cost of non-critical nets. Otherembodiments might use other exponential and non-exponential expressionsin their cost functions. Thus, one of ordinary skill in the art wouldunderstand that the invention is not to be limited by the foregoingillustrative details, but rather is to be defined by the appendedclaims.

We claim:
 1. A method of searching for a global path between first andsecond sets of routable elements in a region of a layout, the methodcomprising: a) partitioning the region into a plurality of rectangularsub-regions; b) identifying a set of sub-regions that contain the twosets of elements; c) performing a path search to identify a set of pathexpansions between a sub-region that contains a first-set element and asub-region that contains a second-set element, wherein performing thepath search comprises exploring expansions along non-Manhattandirections between the sub-regions.
 2. The method of claim 1, whereineach expansion is along an edge that connects two sub-regions, whereinat least some edges are non-Manhattan edges.
 3. The method of claim 2,wherein some edges are Manhattan edges.
 4. The method of claim 3,wherein the layout has multiple layers, and the non-Manhattan edges areon different layers than the Manhattan edges.
 5. The method of claim 1,wherein performing the path search comprises: a) identifying the startof at least one path; b) iteratively identifying a set of expansionsabout previously identified paths until identifying a path that connectsthe two sets.
 6. The method of claim 5 further comprising: wherein eachpath terminates in a sub-region; wherein identifying an expansion abouta path comprises identifying sub-regions that are reachable from thepath's terminating sub-region.
 7. The method of claim 6, wherein somesub-regions are reachable along Manhattan directions, and somesub-regions are reachable along non-Manhattan directions.
 8. The methodof claim 1, wherein each expansion is across an edge that is between twosub-regions, wherein at least some edges are non-Manhattan edges.
 9. Themethod of claim 1, wherein the sub-regions are Gcells.
 10. The method ofclaim 1 further comprising partitioning the region into a plurality ofGcells, wherein each Gcell contains several sub-regions.
 11. The methodof claim 1, wherein the rectangular sub-regions are squares.
 12. Themethod of claim 11, wherein all the square sub-regions have the samesize.
 13. A computer readable medium storing a computer program forsearching for a global path between first and second sets of routableelements in a region of a layout, the computer program comprising setsof instructions for: a) partitioning the region into a plurality ofrectangular sub-regions; b) identifying a set of sub-regions thatcontain the two sets of elements; c) performing a path search toidentify a set of path expansions between a sub-region that contains afirst-set element and a sub-region that contains a second-set element,wherein performing the path-search comprises exploring expansions alongnon-Manhattan directions between the sub-regions.
 14. The computerreadable medium of claim 13, wherein each expansion is along an edgethat connects two sub-regions, wherein at least some edges arenon-Manhattan edges.
 15. The computer readable medium of claim 14,wherein some edges are Manhattan edges.
 16. The computer readable mediumof claim 15, wherein the layout has multiple layers, and thenon-Manhattan edges are on different layers than the Manhattan edges.17. The computer readable medium of claim 13, wherein each expansion isacross an edge that is between two sub-regions, wherein at least someedges are non-Manhattan edges.
 18. The computer readable medium of claim13, wherein the sub-regions are Gcells.
 19. The computer readable mediumof claim 13, wherein the computer program further comprises a set ofinstructions for partitioning the region into a plurality of Gcells,wherein each Gcell contains several sub-regions.
 20. The computerreadable medium of claim 13, wherein the rectangular sub-regions aresquares.